#include <stdio.h>

int fun(int n)
{
	if(n == 1)
	{
		return -1;
	}
	else
	{
		return (n + fun(n-1));
	}
}

int main(int argc,char *argv[])
{
	int x;
	scanf("%d",&x);
	x = fun(x);
	printf("x = %d\n",x);
	
	return 0;
}